A latch is an electronic device that is typically used to store data. An output of a latch may depend not only on its current input, but also on its previous inputs and outputs. Latches may include any of a variety of logic devices such as NAND gates and NOR gates, and the logic gates may include semiconductor devices such as transistors.
A latch or flip-flop (or circuit, more generally) may be used in applications in which the circuit is exposed to harsh environments, such as in space and military applications. However, these circuits may be susceptible to Single Event Effects (SEE). An SEE is a disturbance in an active semiconductor device caused by a single energetic particle. As semiconductor devices become smaller and smaller, transistor threshold voltages decrease. These lower thresholds reduce the charge per node needed to cause errors. As a result, the semiconductor devices become more and more susceptible to transient upsets.
One type of SEE is a single event upset (SEU). An SEU is a radiation-induced error in a semiconductor device caused when charged particles lose energy by ionizing the medium through which they pass, leaving behind a wake of electron-hole pairs. The electron-hole pairs form a parasitic conduction path, which can cause a false transition on a node. The false transition, or glitch, can propagate through the semiconductor device and may ultimately result in the disturbance of a node containing state information, such as an output of a latch, flip-flop, or register.
An SEU may be caused in any of a variety of ways. As an example, an SEU may be caused by ionizing radiation components, such as neutrons, protons, and heavy ions. Such ionizing radiation components are abundant in space and at commercial flight altitudes. As another example, an SEU may be caused by alpha particles from the decay of trace concentrations of uranium and thorium present in some integrated circuit packaging. As yet another example, an SEU may be caused by detonating nuclear weapons. When a nuclear weapon is detonated, intense fluxes of gamma rays, x-rays, and other high-energy particles are created, and these high-energy particles may cause an SEU.
A latch or flip-flop may be susceptible to SEU in any of a variety of ways. As examples, if an input to or transistor in the latch or flip-flop changes state due to an SEU, then the latch or flip flop may produce an erroneous output.